地球是太阳系从内到外的第三颗行星，也是太阳系中直径、质量和密度仅次于的类地行星。它也常常被称作世界。
英语的地球Earth一词来自于古英语及日耳曼语。地球有数44～46亿岁，有一颗天然卫星月球环绕着地球以27.32天的周期转动，而地球以近24小时的周期角速度并且以一年的周期绕行太阳马克斯·沃夫。
迪士尼有同名纪录片。Why the Earth Is Round地球为什么是圆的？Our never-thought to ask question for today is: Why is the earth round? Why not a cube? A cube? No,thats silly. Is it? Why? Because,because really big things like to be round for some reason. Exactly right.And that reason is gravity.今天，我们未曾想要过的问题是：地球为什么是圆的？为什么不是方形的？方形？那过于荒谬了。是吗？为什么？因为因为十分大的东西一般来说都是圆的。完全正确。
这个原因就是重力。Think of it this way. Suppose the earth were a cube. Then some points on its surface would be farther away from the center than others. The corners, for example, would be much farther out than the rest.我们这样想要，假设地球是一个立方体。而地球表面的某些端点离中间或其它端点较为近。
比如，立方体的各个角离得很近。Since gravity works equally on everything, there would be a tendency to pull those uneven places down. The surface would even itself out. The only geometrical object in which every point on the surface is the same distance from the center is a sphere.由于重力对所有物质的起到是大于的，因此不均匀分布的地方不会被向上纳。表面使其本身显得平均值。唯一一个表面上的每个角到中心距离都大于的几何物体就是球体。
But wait a minute,Ive seen pictures of asteroids that have all sorts of crazy shapes.Why arent they round? Only because they arent big enough for gravity to be a determining factor. If an asteroid were to suddenly become as big as a moon, it would indeed rearrange itself gradually into a sphere.等一下，我看完很多行星的图片，它们都是奇形怪状的。它们为什么不是圆的？那只是因为行星太小，重力无法沦为其决定因素。如果行星忽然显得和月亮一样大，它就不会确实重新安排渐渐变为一个球体。
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